The use of implants involves the insertion of a metal fixture into a prepared hole in the bone. During the healing process, the surrounding bone develops an intimate contact with the implant surface and after a suitable time a prosthesis may be attached to the fixture. Such implants are frequently used in dentistry and in cosmetic surgery.
There is a need for a means of clinically observing the quality of the union between the bone and the implant surface. Implant failures can be caused by errors in placement, and premature or inappropriate loading. A non-destructive test, which could be used before loading the implant would help to reduce failures of this type, and would also enable periodic tests to be carried out on implants which are in use to ensure that they are still satisfactory. The test could also provide a quantitative comparison between different implant systems.
X-rays are sometimes used to test the condition of an implant, but they can only show the presence of gross bone loss around the implant. It is also very difficult to monitor the progress of integration over time with x-rays, since it is difficult to reproduce the viewing position and angle with sufficient accuracy. A different sort of test, albeit a crude one, is to tap the structure attached to the implant with a surgical instrument. This test can only distinguish between satisfactory implants and the most grossly defective systems.
When a resonance frequency is used in order to determine the stiffness of the bone-implant interface, the transducers used may have different properties between individual transducers and between different types of transducers.
The difference must be calibrated for, to achieve comparable results from different measurements.
The calibrations are normally made by measuring the resonance frequency on one calibration block, which is determined to give a nominal resonance frequency. The difference to this nominal value is then either subtracted or added to all later measured values with this transducer.
The approach has some disadvantages: it is assumed that all transducers had the same sensitivity, and that the difference between them is only an offset problem, thereby making it possible to only calibrate the relationship between them for one specific stability.
The early studies mainly used the same transducer to follow a specific implant, and the focus was on the change of resonance frequency, and not on the absolute value. It could be difficult however, to compare results between different patients or studies. Another disadvantage is that the frequency scale (measured in Hz) is a bit difficult to communicate, which may be a scale from approximately 3000 Hz to 9000 Hz.
To solve the above mentioned problem, a new index has been established, ISQ (Implant Stability Quotient). ISQ runs from 1 to 100, and is a close to linear mapping of the Hz-scale.
ISQ is defined by a set of calibration blocks with different stability. All transducers are calibrated on these blocks, and the calibration parameters are programmed into the transducer plug. That way, all transducers will give the same ISQ for the same stability, thus making it possible to compare results between different transducers. Also, there is no need to calibrate the transducers before each measurement occasion, since the transducer is carrying the parameters with it.
Mathematically, ISQ is defined by the following equation:ISQ=FR*FR(u+v*L)+FR*(k+n*L)+p+m*L. Where                FR is measured resonance frequency        u, v, k, n, p, m are calibration factors −1 to +1 (resolution 2/32768), and        L is abutment length (for an abutment transducer)The six parameters u, v, k, n, m and p may all be programmed into each transducer plug.        
To determine the relationship between ISQ and frequency (Hz) for the first time, some transducers from the early clinical studies were used. Five calibration blocks with different stability were manufactured, and the spread in resonance frequency between the blocks where equal to the spread seen in the clinical studies. The relationship between resonance frequency and ISQ was then determined to be linear on these blocks with those transducers, and the end points on the block with the lowest stability and the block with the highest stability were decided. The end points were decided in such a way so that 1 ISQ or 100 ISQ corresponds to a stability outside what had been seen in clinical practice.
After the ISQ-values on the five calibration blocks were decided, they were determined to serve as a standard, when all future types of transducers were manufactured.
The resonance frequency of the transducers was measured by feeding a piezoelectric crystal, attached to the transducer beam, with sinusoidal signal which was swept from approximately 3 to 10 kHz during a couple of seconds. Another crystal, attached to the other side of the beam, measures the voltage, and thus the amplitude, during the sweep. When the swept sinus reaches the resonance frequency of the beam, the output from the second crystal has its maximum.
The International Patent Application No. WO 92/18053, relates to a method of testing an implant attached to a bone of a human or animal subject. The method comprises the steps of bringing a member into contact with the implant; detecting at least one resonance frequency of the member when it is in contact with the implant; and interpreting the detected resonance frequency in terms of the degree of attachment of the implant with respect to the bone. However, the method implies using an analyzing unit being in contact with the implant through a wire.
U.S. Pat. No. 3,355,933 and WO 99/46603 relate to measuring arrangements for measuring surface vibrations of a large object. The arrangements presented in these documents are not suitable for small spaces, such as a mouth of a patient.